The present invention relates to a semiconductor device including a surface micromachined membrane and relates to a method of manufacturing the device.
This type of semiconductor device includes a substrate and a membrane. The membrane is formed above the active surface of a semiconductor substrate, where electric elements are formed. A cavity is located between the active surface and the membrane and hermetically sealed. The cavity and the membrane are formed by, for example, stacking two films on the active surface and etching one underlying film through a hole in the other overlying film. As an example of such a semiconductor device, a surface micromachined pressure sensor is known.
A surface micromachined pressure sensor is proposed in JP-A-2001-504994. The method proposed for manufacturing the pressure sensor is shown in FIGS. 1A to 1F. Firstly, as shown in FIG. 1A, a lower electrode 510, which is, for example, a p-type-impurity-diffused layer (p well), is formed in an active surface of a silicon substrate 500, and then a silicon oxide film 520 is formed to cover the lower electrode 510. Then, as shown in FIG. 1B, an impurity-doped polysilicon layer is deposited and patterned out into predetermined shapes. Then, an impurity-doped polysilicon film 530 is completed by forming etching holes 540 in the impurity-doped polysilicon layer using photolithography and etching. The etching holes 540 extend down from the surface of the polysilicon film 530 to the silicon oxide film 520.
Then, as shown in FIG. 1C, the silicon oxide film 520 and the polysilicon film 530 are covered by a resist 550 such that the etching holes 540 are exposed, and the silicon oxide film 520 is partially etched underneath the polysilicon 530 through the etching holes 540 using, for example, hydrofluoric acid (HF) aqueous solution. As a result, a pressure reference chamber 560, or a cavity 560, is formed at the region that the etched portion of the silicon oxide film 520 has occupied.
Then, as shown in FIG. 1D, the resist 550 is stripped off, and a silicon oxide film 570 is deposited in order to hermetically seal the reference chamber 560 by plugging the etching holes 540. Next, as shown in FIG. 1E, electrodes 580 made of, for example, aluminum are formed for electrically accessing the lower electrode 510 and the polysilicon film 530, and a passivation film 590, which is made of, for example, silicon nitride, is formed. A membrane that includes a portion of the polysilicon 530, a portion of the silicon oxide film 570, and a portion of the passivation film 590 is formed above the pressure reference chamber 560.
Next, as shown in FIG. 1F, the passivation film 590 and the silicon oxide film 570 are stripped off at a predetermined portion in order to define a deforming part 505 in the membrane. Since the membrane becomes thinner and deforms more easily at the portion, where the films 570, 590 have been stripped off, than the rest of the membrane, the part of the membrane that is located inside the portion is defined as the deforming part 505, as shown in FIG. 1F.
The reason why the deforming part 505 needs to be defined is that it is difficult to precisely control the dimensions of the membrane because there are no etch stops when the reference chamber 560 is formed by partially etching the silicon oxide film 520, which is a sacrificial layer. The pressure sensor of FIG. 1F functions as a parallel-plate-type capacitive pressure sensor. That is, a capacitor is formed by the polysilicon film 530, which functions an electrode, and the lower electrode 510. When the deforming part 505 is displaced under a pressure, the distance between the polysilicon film 530 and lower electrode 510 changes and the capacitance of the capacitor also changes. Therefore, the pressure is detected by detecting the capacitance change.
Other than the semiconductor device including a surface micromachined membrane, which is represented by the pressure sensor described above, there is also a semiconductor device including a back micromachined membrane. The membrane is formed in the back surface of a semiconductor substrate, which is opposite to the active surface, where electric elements are formed. For example, in the manufacturing process of a back micromachined pressure sensor, a membrane is formed by partially etching a silicon substrate from the back surface, which is opposite to the active surface on which electric elements such as strain gauges and electrodes are formed. A pressure reference chamber, or a cavity, is formed at the region that the etched portion of the substrate has occupied by bonding a glass stand to the back surface of the silicon substrate.
When a back micromachined pressure sensor and a surface micromachined pressure sensor are compared, the pressure reference chamber of the surface micromachined pressure sensor, which is formed by partially etching a sacrificial layer, tends to have a volume more than two orders of magnitude smaller than that of the back micromachined pressure sensor, which is formed by partially etching the silicon substrate of the sensor from the back surface.
Because even a small change in pressure inside the reference chamber significantly affects the sensor characteristics, a hermetic seal is extremely critical in the surface micromachined pressure sensor. The hermeticity of the reference chamber 560 depends strongly on the method for depositing the silicon oxide film 570 to plug the etching holes 540 in the manufacturing step shown in FIG. 1D. As film deposition methods that are compatible with a semiconductor process, chemical vapor deposition (CVD) and physical vapor deposition (PVD) are used to form the silicon oxide film 570. However, the deposition methods have the following problems.
If the silicon oxide film 570 is formed by low pressure CVD, the source species enter the reference chamber 560 through the etching holes 540. As a result, the silicon oxide film 570 is partially deposited inside the reference chamber 560 to change the thickness of the membrane and prevent the membrane from deforming smoothly. In the worst case, the silicon oxide film 570 forms columns in the reference chamber 560 to completely prevent the membrane from deforming.
If the silicon oxide film 570 is deposited by atmospheric pressure CVD, the pressure inside the reference chamber 560 becomes at the atmospheric level. Therefore, the pressure inside the reference chamber 560 changes in response to the ambient temperature according to Boyle-Charles law. As a result, the membrane deforms in response to the ambient temperature, and the sensor output is influenced by the ambient temperature. Especially, when the pressure level to be detected is low, the sensor output is significantly influenced by the ambient temperature.
If the silicon oxide film 570 is formed by plasma CVD (P-CVD), a slit 575 can be generated in the silicon oxide film 570, for example, near a step located at an end the polysilicon film 530, as shown in FIG. 1D. The slit 575 means a crack, flaw, and so on, which narrows the area hermetically sealed by the silicon oxide film 570 and worsens the reliability in the hermetic sealing.
Even if the silicon oxide film 570 is deposited by PVDs such as sputtering, the coverage of the silicon oxide film 570 at the step is as poor as that with P-CVD. Therefore, the area hermetically sealed by the silicon oxide film 570 is narrowed, and the reliability in the hermetic sealing is insufficient.
The problems described above are common to all surface micromachined semiconductor devices, in which a cavity located on the active surface of a semiconductor substrate is hermetically sealed by a membrane, and are not limited to the surface micromachined pressure sensor described above. For example, a thermopile infrared sensor in which the element for detecting infrared has been formed on a membrane can provide a high detection precision as long as high vacuum is maintained in a cavity in order to thermally insulate the membrane. However, if the hermetic sealing of the cavity is not reliable, the membrane can deteriorate in thermal insulation.